Livebearers are defined as fish that do not lay eggs, but instead the females retain them inside their bodies for a period of several days (usually around 20–28 days) and give birth to live babies capable of looking after themselves. Livebearers are known for being especially prolific breeders. Most livebearers belong to the families Poeciliidae or Goodeidae, both of which belong to the order Cyprinodontiformes. The most well known livebearers being the Guppy, Molly and Platy.

There is some discussion over which of the well-known livebearers can crossbreed. The known crosses are as follows:

Generally if the fish do not share a genus, they can definately not crossbreed. Some crosses are fertile and stable such as the Guppy x Endler, but some are rare, sickly and sterile such as Guppy x Platy.